A neutralization enzyme-linked immunosorbent (Nt-ELISA) assay for determination of protective immunity to measles virus was developed and evaluated. This procedure uses the same initial steps as performed to determine antibody titers by seroneutralization (Nt) test. However, a reduction in virus infectivity by neutralizing antibody was determined by quantitation of viral antigen using ELISA. The serum dilution that resulted in neutralization of 50% of infectious virus could be determined from the absorbance values. To be able to screen a large number of specimens, the conditions of the Nt-ELISA test were adjusted such that negative sera for measles antibodies and the positive ones were clearly distinguished on the basis of a single dilution (1:4). This test showed similar sensitivity (88.3%) and equal specificity as the Nt test when screening 136 serum samples from normal subjects. The estimation of protective antibody titers by Nt and Nt-ELISA methods was strongly correlated (correlation coefficient = 0.91). Thus, the measles Nt-ELISA test is rapid, reproducible, sensitive, and specific for detection of protective measles antibodies.
